The present invention is related to cardiac pacing system analyzers which are designed to test the electrical performance of implantable cardiac pulse generators and the associated pacing lead system at the time of pacemaker implantation and during invasive pacemaker trouble-shooting or evaluation procedures. In particular, the present invention is related to a device and method for testing for retrograde conduction in patients to determine if certain types of cardiac pacemakers are contraindicated.
The normal heart controls pumping of the heart through a conduction system which provides electrical stimulating pulses at a rate that is appropriate for the body's needs. The sino-atrial (SA) node is a small knot of cells which is buried in the roof of the right atrium. It is at this point that the contraction of the heart is initiated, and the SA node is, therefore, called the pacemaker of the heart. Lower in the rear wall of the right atrium, and towards the midline of the heart, there is another knot of cells called the atrio-ventricular (A-V) node. The A-V node is connected to a bundle of long cell fibers which pass down and separate the two ventricles. This bundle of fibers is called the Bundle of His.
The Bundle of His splits into two bundle branches called Purkinje fibers. The cells of the SA node have a property of being able to exclude sodium for a period of time when the cells are "polarized." However, there is a constant low leakage of sodium into the cell and gradually the membrane of the cell reaches a point where the membrane potential causes it to "break down" and a rush of sodium into the cell takes place. This rush of sodium into the cell is called depolarization which results in an electrical discharge which causes a wave-like disturbance to spread from the SA node, causing depolarization of the atrial muscles of the heart. The atria then contract to empty blood into ventricles.
There is, however, no direct transmission of the depolarization wave from the atrial muscle to the ventricular muscle. Conduction into the ventricles occurs after firing of the A-V node as a result of the depolarization wave which spreads to the atria. When the atria are empty, and ventricular filling is completed, the A-V node fires, and the impulse is conducted out of the A-V node down through the Bundle of His and into the Purkinje system. This conduction into the ventricle portion of the heart causes all portions of the ventricular myocardium to contract almost simultaneously, which results in a very effective forceful contraction of pumping action.
Because of the unique construction of the cells of the heart and the construction of the conduction system of the heart, it is possible for an impulse to be conducted backward up through the conduction system, as well as forward in the normal fashion. Any reverse conduction through the heart conduction system is called a "retrograde conduction". Although the SA node in a normal heart controls the origin of the pacing synchronization pulses, cardiac cells have the capability of undergoing spontaneous depolarization and, thereby, establishing other sites as pacemaking sites. This will occur, for example, when the SA node is not operating in the proper manner, and a pacing site in a ventricle chamber takes over as the control site, so that there is an ectopic ventricular focus which causes conduction in the reverse manner through the His/Purkinje system.
The A-V junction may also provide an A-V junctional focus which causes retrograde activation of the atria. It is also possible for an artificial cardiac pacemaker to induce depolarization of the ventricle in a fashion such that reverse conduction or retrograde conduction may occur if the heart is not in heart block.
In other words, if the conduction channel of the His/Purkinje system is operative, a retrograde impulse can initiate atrial contractions which may result in the implanted cardiac pacemaker inducing tachycardias at the upper pacing rate limit of the artificial pacemaker when the patient has a dual chamber pacemaker with an atrial refractory time that is shorter than the patient's retrograde conduction time. Since the dual chamber pacemaker is becoming increasingly favored among cardiologists because of its improved physiologic properties which allows the patient to lead a more normal life, the problem of detecting the potential that such a pacemaker may have for inducing tachycardia of this type is a major consideration of the cardiac pacemaker field.
In a pacemaker in which there is atrial pacing, it is possible for the patient's retrograde conduction time to be longer than the atrial refractory time of the pacemaker. The atrial refractory time is the time that the atrial sense amplifier is deactivated and, therefore, is not capable of sensing pulses which are interpreted by the pacemaker as atrium, or P-wave, pulses. A short atrial refractory time of pacing of the ventricle chamber can cause a retrograde conducted depolarization wave to be sent by the atrial sense amplifier, which can cause the pacemaker to fire another ventricle pulse that will be sensed thereby eventually resulting in pacemaker induced tachycardia by forcing the pacemaker to go to its upper rate limit.
Modern dual chamber physiologic pacemakers are generally capable of operating either in the atrial synchronous, ventricular-inhibited (VDD) or the ventricular demand (VVI) modes. The present invention is useful when preliminary indications indicate demand pacing mode may be proper for the patient, but before implantation is complete, the physician wishes to ensure that the pacemaker will not itself produce tachycardia because of an abnormally long patient retrograde conduction time. The invention is also useful in universal, or DDD pacemakers which have provisions for intracardiac electrograms.